It looks like you answered your own question as I was creating the screenshots. You can set custom to any size you like, it saves automatically to your settings (unless you're using the trial and then it may time out before they're stored). It should switch almost instantly, even on my $60 Win10 tablet it updates in less than 0.5 second.

@Zax When i try to make this grid to burn i followed everything to the letter but i only get horizonta lrectangles i dont get ant vertical lines to make a grid pattern any ideas to what this could be i am using the most up to date version 1.04j
thankyou